Injection molding industry in the current market is more diverse under the premise, but also continue to develop and broaden, there have been such as multi-color injection, gas-assisted, in-mold foil, co-injection molding and other new processes. , But also in two directions - large tonnage injection molding machines and micro-injection molding machines are constantly upgrading, this article focuses on the author's view on the micro-injection molding process.
In recent years, there has been an increasing demand for miniature products. There has been a large demand for miniature injection molded parts in the electronics industry, watch industry and military industry. These injection molded products are very demanding in size and precision.
Under this kind of premise, the miniature injection molding process also faces the huge challenge, how to make the injection molding to satisfy the micron level size request, simultaneously also must have the good appearance and the performance? Here we are regarding the miniature injection molding in the mold, the equipment, Materials, technology and other aspects of the traditional injection molding to do a brief introduction.
Mold processing and points
In terms of molds, the requirements for processing equipment for micro-injection molding are much higher than conventional injection molding.
Micro-injection molding in the mold usually have two trends: The first is the use of mirror-spark machining, in order to ensure high precision, it is best to use graphite electrodes for EDM, because the graphite electrode loss than the average copper electrode A lot smaller
The second most commonly used processing method is the use of electroforming mold, the use of electroforming process can ensure very high accuracy, but the disadvantage is the long processing cycle, each hole must be independently processed, there are minor damage in the production can not be repaired , Only to replace the damaged acupuncture points.
In terms of mold, mold temperature is also a very important parameter for miniature injection molding.For the high-end customers, the most common practice at present is to borrow the concept of high-gloss plastic injection into the rapid heating and cooling system.
Theoretically, high mold temperature is very helpful for miniature injection molding. For example, it can prevent the difficulty of thin-wall filling and lack of material, but too high mold temperature brings new problems such as longer period and shrinkage after opening.
Therefore, the introduction of new mold temperature control system is quite important in the injection molding process can increase the mold temperature (which can exceed the melting point of the plastic used), so that the melt can quickly fill the cavity to prevent the melt in the filling process to reduce the temperature over Quickly cause incomplete filling; and when the mold release, the mold temperature can be rapidly reduced to maintain a slightly lower temperature than the plastic heat distortion temperature, and then open the mold ejection.
In addition, the micro-injection molding is the quality of milligrams of products, if the use of ordinary casting system for injection molding products, even after the most optimized improvements, products and casting system, the mass ratio of the material is still 1: 10. Less than 10% of the material is injection molded into micro-products, resulting in a large amount of casting system condensate, so micro-injection should be hot runner pouring system.
Material selection points
Material selection, it is recommended in the early stages of development can choose some low viscosity, good thermal stability of the common engineering plastics.
The choice of low-viscosity materials is due to the low viscosity of the melt during filling, the relatively low resistance of the entire casting system and the faster filling rate, which ensures that the melt will fill the cavity smoothly and the melt temperature will not be significantly reduced , Otherwise it is easy to form a cold seam on the product, and the molecules in the filling process less orientation, the resulting product performance is more uniform.
If the choice of high-viscosity plastic, not only slow filling, and feeding time longer, due to feed shear flow easily lead to the molecular chain in the shear flow direction, in this case cooling below the softening point orientation state Is frozen, and this kind of frozen orientation tends to cause internal stress of the product, even causing stress cracking or warpage of the product.
Require good thermal stability of plastic is due to the material stay in the hot runner for a long time or by the shear effect of the screw easily lead to thermal degradation, especially for heat-sensitive plastic, even in a very short cycle time, but also because of material injection The small amount of residence time in the casting system is relatively long, resulting in a considerable degree of plastic degradation, so the heat-sensitive plastic is not suitable for micro-injection.
Equipment selection points
In equipment selection, the size of the micro-injection molded parts of micron-sized products, injection volume should be selected for the milligram-level injection machine.
Injection unit of this type of injection molding machine generally adopts the combination of screw and plunger, the plasticization of material is completed by the screw, and the melt is injected into the cavity by the plunger. The screw plunger injection molding machine can combine the high Accuracy and high speed plunger equipment, to ensure the accuracy of the production process and filling speed.
In addition, the injection molding machine is usually composed of a mold guide, an injection system, a pneumatic ejection mechanism, a quality inspection mechanism, and an automatic packaging system. A good quality inspection system ensures the yield of the micro-precision injection molded product and monitors the entire process Parameter fluctuations.
Injection molding process points
Finally, we look at the requirements of the micro-injection molding technology in the injection molding process, we need to consider the gas gate marks and stress problems, usually require the use of multi-level injection molding to ensure that the material can be steady flow state filling.
In addition, the need to consider the size of dwell time, too small packing pressure will lead to product shrinkage, but too much pressure will lead to stress concentration, as well as larger size and other issues.
In addition, the material in the material pipe to stay in the time also need to be strictly monitored, the material in the material pipe to stay too long, will lead to material degradation affect the function of the product.It is recommended to process parameters in the management of the standard parameters, each Products in the best pre-production DOE verification can do, all changes in production have to re-size and function testing.
As a branch in the field of injection molding, miniature injection molding is moving towards the requirements of high dimensional accuracy, high functional requirements and high appearance. Only through the strict control and continuous improvement of various processes such as mold, equipment, materials and processes, Field development.